Signal generator

The present invention relates to a phase accumulation type signal generator which accumulates an amount of phase to be added (hereinafter referred to as a phase add amount), for each clock, reads data out of a waveform memory using the added output, and converts the read output into an analog signal.

FIG. 1 is a block diagram of a conventional phase accumulation type signal generator. In a phase accumulator 11 a phase add amount n and the output k of a latch circuit 13 are added by an adder 12 and the added output n+k is latched in the latch circuit 13 for each clock, whereby the phase add amount n is accumulated. A signal of a frequency f, produced by an oscillator 14, is frequency divided by a variable frequency divider 15 down to 1/m to yield a clock, by which the latch circuit 13 is latched.

Data is read out of a waveform memory 16 using the output of the latch circuit 13, and the read output of the waveform memory 16 is latched by the clock in a latch circuit 17. The output of the latch circuit 17 is converted by a D-A converter 18 into an analog signal, which is applied to a low-pass filter 19, an amplifier 21 and an attenuator 22.

The phase accumulation type signal generator in FIG. 1 is capable of generating, with a simple arrangement, low-distortion signals of high frequency accuracy at low and even ultra-low frequencies. However, when the phase add amount n does not bear an integral multiple relationship to the maximum value of the adder 12 (that is, the address space of the memory 16), the amount of offset of the phase (hereinafter referred to as a phase offset amount) at the time a carry is generated, will vary. For example, as shown in FIG. 2A, the phase add amount n is accumulated for each clock, the output of the adder 12 increases as shown in FIG. 2B, and upon generation of the clock immediately after an overflow of the adder 12, a carry is yielded as a synchronization pulse as depicted in FIG. 2C. The phase offset amount Δφ of this synchronization pulse fluctuates as shown in FIG. 2D, and consequently, such a synchronization pulse cannot be employed for triggering a synchronous detection or Fourier transformation for impedance measurement.

Impedance measurement calls for a sine-wave signal which is applied to a measuring object and a pulse signal which is accurately synchronized with the sine-wave signal. FIG. 3 shows a conventional method used to meet this requirement. The output of an oscillator 31 is frequency divided by a frequency divider 32, the output of which is filtered by a low-pass filter 33 to produce a sine-wave signal. The sine-wave signal is applied to a measuring object 34, the current output of which is converted by a current-voltage converter 35 into a voltage signal. The voltage signal is provided to a synchronous detector 36, wherein it is synchronously detected by the output of the frequency divider 32, and the synchronously detected output is integrated by an integrator 37.

In this instance, since the filter characteristic of the low-pass filter 33 is fixed, the number of frequencies used is limited to one or several and the frequency used cannot be changed.

It is therefore an object of the present invention to provide a phase accumulation type signal generator with which it is possible to obtain a synchronization pulse accurately synchronized with a waveform which is provided from a waveform memory.

According to the phase accumulation type signal generator of the present invention, when a carry output is yielded from the adder of the phase accumulator, an analog voltage is generated which is in proportion to the difference between the adder output k s and the phase add amount n, and a ramp function voltage V(t)=nfa 1 a 3 t/(cm) is produced which is in proportion to the product of the clock frequency f/m and the phase add amount n. The analog voltage and the ramp function voltage are compared by a comparator, which yields a synchronization pulse when they agree with each other.

That is, the synchronization pulse is generated at t=a 2 (n=k s )cm/(nfa 1 a 3 ). If it is predetermined that a 2 c/(a 1 a 3 )=1, then the synchronization pulse is always fixed to a phase offset 2m/f (sec).

FIG. 1 is a block diagram of a conventional signal generator;

FIGS. 2A, 2B, 2C and 2D show waveforms for explaining the operation of the conventional signal generator;

FIGS. 2E and 2F are waveforms for explaining the operation of the present invention;

FIG. 3 is a block diagram of a conventional impedance measuring apparatus;

FIG. 4 is a block diagram of an embodiment of the signal generator of the present invention;

FIG. 5 is a block diagram of a specific operative example of a difference circuit 41 used in FIG. 4;

FIG. 6 is a block diagram of another example of the difference circuit 41; and

FIG. 7 is a block diagram of a modified form of the signal generator of the present invention.

FIG. 4 illustrates in block form an embodiment of the present invention, in which the parts corresponding to those in FIG. 1 are identified by the same reference numerals. According to the present invention, when a carry output is yielded from the adder 12, a difference circuit 41 generates an analog voltage V 1 =a 2 (n-k s ) which is in proportion to the difference between the output k s of the adder 12 and the phase add amount n. The carry output from the adder 12 is loaded into a D-type flip-flop 42 by the clock from the frequency divider 15. The output of the D-type flip-flop 42 is applied to an input Sync of the difference circuit 41. The difference circuit 41 is supplied with the phase add amount n and the output k s of the adder 12.

FIG. 5 shows a specific operative example of the difference circuit 41, in which the input phase add amount n thereto is converted by a D-A converter 43 into an analog voltage a 2 n, the output k s of the adder 12 is latched by the Sync input in a latch circuit 44, and the output k s of the latch circuit 44 is converted by a D-A converter 45 into an analog voltage a 2 k s , which is subtracted from the analog voltage a 2 n in an analog subtractor 46, providing a 2 (n-k s ).

FIG. 6 shows another example of the difference circuit 41, in which the output k s of the adder 12 is reversed in sign by a NOT circuit 47 to obtain a complement k s which is provided to an adder 48, wherein it is added with the phase add amount n and 1 to obtain n+k s +1. The added output is latched in a latch circuit 49 by the Sync input. The latch output is converted by a multiplier type D-A converter 51 into the analog voltage a 2 (n-k s ).

In FIG. 4, a ramp function voltage V(t)=nfa 1 a 3 t/cm is produced which is in proportion to the product of the clock frequency f/m and the phase add amount n. Consequently, a value nfa 1 /m which is in proportion to the product of the clock frequency f/m and the phase add amount n is converted by a D-A converter 52 into an analog current i=-nfa 1 a 3 /m. This analog current is integrated by an integrator 50. An inverted signal of the output of the D-type flip-flop 42 is applied to a D-type flip-flop 53 to load therein a high-level signal "1", and a low-level signal from the Q output of the D-type flip-flop 53 is applied to a reset switch 54 of the integrator 50, turning it OFF. Consequently, when the synchronization pulse, i.e. the carry shown in FIG. 2C, falls, the Q output of the flip-flop 53 goes to a "0", by which the reset switch 54 is turned OFF, and the integrator 50 starts integration as shown in FIG. 2E, yielding a ramp function voltage as follows: ##EQU1## The D-A converter 52 and the integrator 50 constitute a ramp function voltage generator 60. The ramp function voltage V(t) and the analog voltage V 1 from the difference circuit 41 are compared by a comparator 55, which yields a high-level signal "1" when the ramp function voltage V(t) exceeds the analog voltage V 1 . The output of the comparator 55 is applied via a changeover switch 56 to an output terminal 57, and at the same time, it is applied to a reset terminal R of the D-type flip-flop 53. When the output of the comparator 55 goes high-level, the flip-flop 53 is reset and its Q output also goes high-level, by which the reset switch 54 is turned ON, resetting the integrator 50. As a result of this, the output of the comparator 55 becomes as shown in FIG. 2F.

Based on time resolution m/f of the phase accumulation clock, the phase add amount n and the output k s of the adder 12 at the time of generation of its carry output, the phase offset time becomes (n-k s )f/nm seconds. The comparator 55 yields a pulse at a time point t=a 2 (n-k s )cm/nfa 1 a 3 when the ramp function voltage V(t) becomes equal to the analog voltage V 1 . In this instance, if it is determined that a 2 c/a 1 a 3 =1, then the output pulse from the comparator 55 will become a synchronization pulse offset Δφ by a 2m/f (sec) phase, that is, a fixed-phase-offset synchronization pulse. The synchronization pulse from the comparator 55 and the synchronization pulse from the flip-flop 42 can selectively be provided via the switch 56 as required.

It is also possible to generate two or more synchronization pulses at regular time intervals every cycle of the waveform which is read out of the waveform memory 16. FIG. 7 illustrates a part of the circuit arrangement therefor, in which the parts corresponding to those in FIG. 4 are indicated by the same reference numerals. The carry output of the adder 12 is counted by a counter 61, the content of which is latched as a high-order bit in the latch circuit 13. The contents of the counter 13 latched in the latch circuit 13 and the adder 12 are used to read data from the waveform memory 16. When the counter 61 is a 1-bit counter, two synchronization pulses are yielded from the comparator 55 at regular time intervals during one cycle of the waveform which is read out of the waveform memory 16. When the counter 61 is a 2-bit counter, the comparator 55 generates four synchronization pulses at regular time intervals during one cycle of the waveform which is read out of the waveform memory 16.

As described above, according to the present invention, a synchronization pulse of a fixed phase offset amount Δφ can be obtained even if the phase add amount n does not bear an integral multiple relationship to the address space of the waveform memory 16. It is therefore possible, for example, in an impedance measuring apparatus, to supply a measuring object with a sine-wave signal obtained from the waveform memory 16 and synchronously detect the output of the measuring object by a synchronization pulse. In addition, the frequency of the waveform signal can easily be changed by changing the phase add amount n or the frequency dividing ratio m of the frequency divider 15. For instance, signals of frequencies 10 Hz to 1 MHz can be generated with a resolution of 1 mHz.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.